This invention relates generally to repair of cracks in superalloys, and more particularly to methods for achieving enhanced compatibility between a braze compound and the article parent material, reduced distortion of the parent material, and for achieving high temperature properties of the final braze close to those of the parent material.
Superalloys are employed in articles such as gas turbine vanes because they exhibit high strength at high temperatures. Typical superalloys are nickle based alloys (as for example Rene 80), or cobalt based alloys (X-40). Despite such desirable properties, superalloys can and do become cracked, in high temperature operation such as in gas turbine engines. It then becomes necessary or desirable to repair the cracks. It is further found that complex oxides, including one or both of Ti and Al, occur, particularly in the case of nickel based superalloy, and at the crack surfaces. Complex oxides also occur at crack surfaces in cobalt based superalloys. Such oxides must be removed in order to effectively repair the cracks. One approach has been to attempt removal of oxides through exposure of same to reducing atmospheres, such as hydrogen, in a furnace environment; however, this approach has been found impractical as a sole means to remove oxides prior to braze repair of cracks, as recognized in U.S. Pat. No. 4,098,450 to Keller. Further, full penetration of narrow cracks by the melted braze is not always assured. Treatment of nickel superalloys with active fluoride ions to remove oxides in cracks is disclosed in U.S. Pat. No. 4,098,450; however, the problem of subsequent, adequate braze penetration of cracks remains, and oxide removal is not satisfactorily assured and is not inspectable, nondestructively.